Require an icon to be specified on icon mappings

[mollykreis]

Pull Request

🤨 Rationale

In #1874, we introduced the ability to have icon mappings with an undefined icon to support use cases where the cell should be empty with non-empty group rows and where the cell should have only text. These use cases are now covered by usages of nimble-mapping-empty and nimble-mapping-text, both of which are supported by the mapping column. Therefore, we no longer need to support icon mappings without an icon.

👩‍💻 Implementation

Mostly reverting #1874, with a few minor updates to account for updates made in the source since the change was put it.

🧪 Testing

• Added new unit test that verifies setting an undefined icon on an icon mapping in the mapping column results in a validation error
• Updated matrix tests

✅ Checklist

• [ ] I have updated the project documentation to reflect my changes or determined no changes are needed.

[mollykreis]

@m-akinc, will you buddy this PR for me?